Changes required to make prefork clean up idle children properly. There was a window...

Changes required to make prefork clean up idle children properly. There was a window during which a starting worker deadlocks when an idle cleanup arrives before it completes init. Apache then keeps trying to cleanup the same deadlocked worker forever (until higher pids come along, but it still will never reduce below the deadlocked pid). Thus the number of children would not reduce to the correct idle level.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@88741 13f79535-47bb-0310-9956-ffa450edef68